Computer Architecture: A Quantitative Approach, 3rd Edition, 2002

1142 Pages·2002·4.81 MB·English

by  John L. Hennessy

1
Fundamentals of Computer Design
And now for something completely different.
Monty Python’s Flying Circus
1.1 Introduction 1
1.2 The Task of a Computer Designer 4
1.3 Technology Trends 11
1.4 Cost, Price and their Trends 14
1.5 Measuring and Reporting Performance 25
1.6 Quantitative Principles of Computer Design 40
1.7 Putting It All Together: Performance and Price-Performance 49
1.8 Another View: Power Consumption and Efficiency as the Metric 58
1.9 Fallacies and Pitfalls 59
1.10 Concluding Remarks 69
1.11 Historical Perspective and References 70
Exercises 77
1.1
Introduction
Computer technology has made incredible progress in the roughly 55 years since
the first general-purpose electronic computer was created. Today, less than a
thousand dollars will purchase a personal computer that has more performance,
more main memory, and more disk storage than a computer bought in 1980 for
$1 million. This rapid rate of improvement has come both from advances in the
technology used to build computers and from innovation in computer design.
Although technological improvements have been fairly steady, progress aris-
ing from better computer architectures has been much less consistent. During the
first 25 years of electronic computers, both forces made a major contribution; but
beginning in about 1970, computer designers became largely dependent upon in-
tegrated circuit technology. During the 1970s, performance continued to improve
at about 25% to 30% per year for the mainframes and minicomputers that domi-
nated the industry.
The late 1970s saw the emergence of the microprocessor. The ability of the
microprocessor to ride the improvements in integrated circuit technology more
closely than the less integrated mainframes and minicomputers led to a higher
rate of improvement—roughly 35% growth per year in performance.
2 Chapter 1 Fundamentals of Computer Design
This growth rate, combined with the cost advantages of a mass-produced
microprocessor, led to an increasing fraction of the computer business being
based on microprocessors. In addition, two significant changes in the computer
marketplace made it easier than ever before to be commercially successful with a
new architecture. First, the virtual elimination of assembly language program-
ming reduced the need for object-code compatibility. Second, the creation of
standardized, vendor-independent operating systems, such as UNIX and its
clone, Linux, lowered the cost and risk of bringing out a new architecture.
These changes made it possible to successfully develop a new set of architec-
tures, called RISC (Reduced Instruction Set Computer) architectures, in the early
1980s. The RISC-based machines focused the attention of designers on two criti-
cal performance techniques, the exploitation of instruction-level parallelism (ini-
tially through pipelining and later through multiple instruction issue) and the use
of caches (initially in simple forms and later using more sophisticated organiza-
tions and optimizations). The combination of architectural and organizational en-
hancements has led to 20 years of sustained growth in performance at an annual
rate of over 50%. Figure 1.1 shows the effect of this difference in performance
growth rates.
The effect of this dramatic growth rate has been twofold. First, it has signifi-
cantly enhanced the capability available to computer users. For many applica-
tions, the highest performance microprocessors of today outperform the
supercomputer of less than 10 years ago.
Second, this dramatic rate of improvement has led to the dominance of micro-
processor-based computers across the entire range of the computer design. Work-
stations and PCs have emerged as major products in the computer industry.
Minicomputers, which were traditionally made from off-the-shelf logic or from
gate arrays, have been replaced by servers made using microprocessors. Main-
frames have been almost completely replaced with multiprocessors consisting of
small numbers of off-the-shelf microprocessors. Even high-end supercomputers
are being built with collections of microprocessors.
Freedom from compatibility with old designs and the use of microprocessor
technology led to a renaissance in computer design, which emphasized both ar-
chitectural innovation and efficient use of technology improvements. This renais-
sance is responsible for the higher performance growth shown in Figure 1.1—a
rate that is unprecedented in the computer industry. This rate of growth has com-
pounded so that by 2001, the difference between the highest-performance micro-
processors and what would have been obtained by relying solely on technology,
including improved circuit design, is about a factor of fifteen.
In the last few years, the tremendous imporvement in integrated circuit capa-
bility has allowed older less-streamlined architectures, such as the x86 (or IA-32)
architecture, to adopt many of the innovations first pioneered in the RISC de-
signs. As we will see, modern x86 processors basically consist of a front-end that
fetches and decodes x86 instructions and maps them into simple ALU, memory
access, or branch operations that can be executed on a RISC-style pipelined pro-
1.1 Introduction 3
350
DEC Alpha
300
250 1.58x per year
200
SPECint rating
DEC Alpha
150
IBM Power2
DEC Alpha
100
1.35x per year
HP
9000
50 MIPS IBM
MIPS Power1
R3000
R2000
SUN4
0
1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995
Year
FIGURE 1.1 Growth in microprocessor performance since the mid 1980s has been substantially higher than in ear-
lier years as shown by plotting SPECint performance. This chart plots relative performance as measured by the SPECint
benchmarks with base of one being a VAX 11/780. (Since SPEC has changed over the years, performance of newer ma-
chines is estimated by a scaling factor that relates the performance for two different versions of SPEC (e.g. SPEC92 and
SPEC95.) Prior to the mid 1980s, microprocessor performance growth was largely technology driven and averaged about
35% per year. The increase in growth since then is attributable to more advanced architectural and organizational ideas. By
2001 this growth leads to about a factor of 15 difference in performance. Performance for floating-point-oriented calculations
has increased even faster.
Change this figure as follows:
!1. the y-axis should be labeled “Relative Performance.”
2. Plot only even years
3. The following data points should changed/added:
a. 1992 136 HP 9000; 1994 145 DEC Alpha; 1996 507 DEC Alpha; 1998 879 HP 9000; 2000 1582 Intel
Pentium III
4. Extend the lower line by increasing by 1.35x each year
4 Chapter 1 Fundamentals of Computer Design
cessor. Beginning in the end of the 1990s, as transistor counts soared, the over-
head in transistors of interpreting the more complex x86 architecture became
neglegible as a percentage of the total transistor count of a modern microproces-
sor.
This text is about the architectural ideas and accompanying compiler improve-
ments that have made this incredible growth rate possible. At the center of this
dramatic revolution has been the development of a quantitative approach to com-
puter design and analysis that uses empirical observations of programs, experi-
mentation, and simulation as its tools. It is this style and approach to computer
design that is reflected in this text.
Sustaining the recent improvements in cost and performance will require con-
tinuing innovations in computer design, and the authors believe such innovations
will be founded on this quantitative approach to computer design. Hence, this
book has been written not only to document this design style, but also to stimu-
late you to contribute to this progress.
1.2
The Changing Face of Computing and the
Task of the Computer Designer
In the 1960s, the dominant form of computing was on large mainframes, ma-
chines costing millions of dollars and stored in computer rooms with multiple op-
erators overseeing their support. Typical applications included business data
processing and large-scale scientific computing. The 1970s saw the birth of the
minicomputer, a smaller sized machine initially focused on applications in scien-
tific laboratories, but rapidly branching out as the technology of timesharing,
multiple users sharing a computer interactively through independent terminals,
became widespread. The 1980s saw the rise of the desktop computer based on
microprocessors, in the form of both personal computers and workstations. The
individually owned desktop computer replaced timesharing and led to the rise of
servers, computers that provided larger-scale services such as: reliable, long-term
file storage and access, larger memory, and more computing power. The 1990s
saw the emergence of the Internet and the world-wide web, the first successful
handheld computing devices (personal digital assistants or PDAs), and the emer-
gence of high-performance digital consumer electronics, varying from video
games to set-top boxes.
These changes have set the stage for a dramatic change in how we view com-
puting, computing applications, and the computer markets at the beginning of the
millennium. Not since the creation of the personal computer more than twenty
years ago have we seen such dramatic changes in the way computers appear and
in how they are used. These changes in computer use have led to three different
computing markets each characterized by different applications, requirements,
and computing technologies.
1.2 The Changing Face of Computing and the Task of the Computer Designer 5
Desktop Computing
The first, and still the largest market in dollar terms, is desktop computing. Desk-
top computing spans from low-end systems that sell for under $1,000 to high-
end, heavily-configured workstations that may sell for over $10,000. Throughout
this range in price and capability, the desktop market tends to be driven to opti-
mize price-performance. This combination of performance (measured primarily
in terms of compute performance and graphics performance) and price of a sys-
tem is what matters most to customers in this market and hence to computer de-
signers. As a result desktop systems often are where the newest, highest
performance microprocessors appear, as well as where recently cost-reduced mi-
croprocessors and systems appear first (see section 1.4 on page 14 for a discus-
sion of the issues affecting cost of computers).
Desktop computing also tends to be reasonably well characterized in terms of
applications and benchmarking, though the increasing use of web-centric, inter-
active applications poses new challenges in performance evaluation. As we dis-
cuss in Section 1.9 (Fallacies, Pitfalls), the PC portion of the desktop space seems
recently to have become focused on clock rate as the direct measure of perfor-
mance, and this focus can lead to poor decisions by consumers as well as by de-
signers who respond to this predilection.
Servers
As the shift to desktop computing occurred, the role of servers to provide larger
scale and more reliable file and computing services grew. The emergence of the
world-wide web accelerated this trend due to the tremendous growth in demand
for web servers and the growth in sophistication of web-based services. Such
servers have become the backbone of large-scale enterprise computing replacing
the traditional mainframe.
For servers, different characteristics are important. First, availability is critical.
We use the term availability, which means that the system can reliably and effec-
tively provide a service. This term is to be distinguished from reliability, which
says that the system never fails. Parts of large-scale systems unavoidably fail; the
challenge in a server is to maintain system availability in the face of component
failures, usually through the use of redundancy. This topic is discussed in detail
in Chapter 6.
Why is availability crucial? Consider the servers running Yahoo!, taking or-
ders for Cisco, or running auctions on EBay. Obviously such systems must be op-
erating seven days a week, 24 hours a day. Failure of such a server system is far
more catastrophic than failure of a single desktop. Although it is hard to estimate
the cost of downtime, Figure 1.2 shows one analysis, assuming that downtime is
distributed uniformly and does not occur solely during idle times. As we can see,
the estimated costs of an unavailable system are high, and the estimated costs in
6 Chapter 1 Fundamentals of Computer Design
Figure 1.2 are purely lost revenue and do not account for the cost of unhappy cus-
tomers!
Application Cost of downtime Annual losses (millions of $) with downtime of
per hour
(thousands of $) 1% 0.5% 0.1%
(87.6 hrs/yr) (43.8 hrs/yr) (8.8 hrs/yr)
Brokerage operations $6,450 $565 $283 $56.5
Credit card authorization $2,600 $228 $114 $22.8
Package shipping services $150 $13 $6.6 $1.3
Home shopping channel $113 $9.9 $4.9 $1.0
Catalog sales center $90 $7.9 $3.9 $0.8
Airline reservation center $89 $7.9 $3.9 $0.8
Cellular service activation $41 $3.6 $1.8 $0.4
On-line network fees $25 $2.2 $1.1 $0.2
ATM service fees $14 $1.2 $0.6 $0.1
FIGURE 1.2 The cost of an unavailable system is shown by analyzing the cost of downtime (in terms of immedi-
ately lost revenue), assuming three different levels of availability. This assumes downtime is distributed uniformly. This
data is from Kembel [2000] and was collected an analyzed by Contingency Planning Research.
A second key feature of server systems is an emphasis on scalability. Server
systems often grow over their lifetime in response to a growing demand for the
services they support or an increase in functional requirements. Thus, the ability
to scale up the computing capacity, the memory, the storage, and the I/O band-
width of a server are crucial.
Lastly, servers are designed for efficient throughput. That is, the overall per-
formance of the server–in terms of transactions per minute or web pages served
per second–is what is crucial. Responsiveness to an individual request remains
important, but overall efficiency and cost-effectiveness, as determined by how
many requests can be handled in a unit time, are the key metrics for most servers.
(We return to the issue of performance and assessing performance for different
types of computing environments in Section 1.5 on page 25).
Embedded Computers
Embedded computers, the name given to computers lodged in other devices
where the presence of the computer is not immediately obvious, are the fastest
growing portion of the computer market. The range of application of these devic-
es goes from simple embedded microprocessors that might appear in a everyday
machines (most microwaves and washing machines, most printers, most net-
working switches, and all cars contain such microprocessors) to handheld digital
devices (such as palmtops, cell phones, and smart cards) to video games and digi-
tal set-top boxes. Although in some applications (such as palmtops) the comput-
1.2 The Changing Face of Computing and the Task of the Computer Designer 7
ers are programmable, in many embedded applications the only programming
occurs in connection with the initial loading of the application code or a later
software upgrade of that application. Thus, the application can usually be careful-
ly tuned for the processor and system; this process sometimes includes limited
use of assembly language in key loops, although time-to-market pressures and
good software engineering practice usually restrict such assembly language cod-
ing to a small fraction of the application. This use of assembly language, together
with the presence of standardized operating systems, and a large code base has
meant that instruction set compatibility has become an important concern in the
embedded market. Simply put, like other computing applications, software costs
are often a large factor in total cost of an embedded system.
Embedded computers have the widest range of processing power and cost.
From low-end 8-bit and 16-bit processors that may cost less than a dollar, to full
32-bit microprocessors capable of executing 50 million instructions per second
that cost under $10, to high-end embedded processors (that can execute a billion
instructions per second and cost hundreds of dollars) for the newest video game
or for a high-end network switch. Although the range of computing power in the
embedded computing market is very large, price is a key factor in the design of
computers for this space. Performance requirements do exist, of course, but the
primary goal is often meeting the performance need at a minimum price, rather
than achieving higher performance at a higher price.
Often, the performance requirement in an embedded application is a real-time
requirement. A real-time performance requirement is one where a segment of the
application has an absolute maximum execution time that is allowed. For exam-
ple, in a digital set-top box the time to process each video frame is limited, since
the processor must accept and process the next frame shortly. In some applica-
tions, a more sophisticated requirement exists: the average time for a particular
task is constrained as well as the number of instances when some maximum time
is exceeded. Such approaches (sometimes called soft real-time) arise when it is
possible to occasionally miss the time constraint on an event, as long as not too
many are missed. Real-time performance tend to be highly application depen-
dent. It is usually measured using kernels either from the application or from a
standardized benchmark (see the EEMBC benchmarks described in Section 1.5).
With the growth in the use of embedded microprocessors, a wide range of bench-
mark requirements exist, from the ability to run small, limited code segments to
the ability to perform well on applications involving tens to hundreds of thou-
sands of lines of code.
Two other key characteristics exist in many embedded applications: the need
to minimize memory and the need to minimize power. In many embedded appli-
cations, the memory can be substantial portion of the system cost, and memory
size is important to optimize in such cases. Sometimes the application is expected
to fit totally in the memory on the processor chip; other times the applications
needs to fit totally in a small off-chip memory. In any event, the importance of
memory size translates to an emphasis on code size, since data size is dictated by
8 Chapter 1 Fundamentals of Computer Design
the application. As we will see in the next chapter, some architectures have spe-
cial instruction set capabilities to reduce code size. Larger memories also mean
more power, and optimizing power is often critical in embedded applications. Al-
though the emphasis on low power is frequently driven by the use of batteries, the
need to use less expensive packaging (plastic versus ceramic) and the absence of
a fan for cooling also limit total power consumption.We examine the issue of
power in more detail later in the chapter.
Another important trend in embedded systems is the use of processor cores to-
gether with application-specific circuitry. Often an application’s functional and
performance requirements are met by combining a custom hardware solution to-
gether with software running on a standardized embedded processor core, which
is designed to interface to such special-purpose hardware. In practice, embedded
problems are usually solved by one of three approaches:
1. using a combined hardware/software solution that includes some custom hard-
ware and typically a standard embedded processor,
2. using custom software running on an off-the-shelf embedded processor, or
3. using a digital signal processor and custom software. (Digital signal proces-
sors are processors specially tailored for signal processing applications. We
discuss some of the important differences between digital signal processors
and general-purpose embedded processors in the next chapter.)
Most of what we discuss in this book applies to the design, use, and performance
of embedded processors, whether they are off-the-shelf microprocessors or mi-
croprocessor cores, which will be assembled with other special-purpose hard-
ware. The design of special-purpose application-specific hardware and the
detailed aspects of DSPs, however, are outside of the scope of this book.
Figure 1.3 summarizes these three classes of computing environments and
their important characteristics.
The Task of a Computer Designer
The task the computer designer faces is a complex one: Determine what
attributes are important for a new machine, then design a machine to maximize
performance while staying within cost and power constraints. This task has many
aspects, including instruction set design, functional organization, logic design,
and implementation. The implementation may encompass integrated circuit de-
sign, packaging, power, and cooling. Optimizing the design requires familiarity
with a very wide range of technologies, from compilers and operating systems to
logic design and packaging.
In the past, the term computer architecture often referred only to instruction
set design. Other aspects of computer design were called implementation, often